This invention relates to magnetic flowmeters, and in particular to such flowmeters in which the pick-up electrodes are provided with ultrasonic cleaners.
In an electromagnetic flowmeter a magnetic field across a flow tube generates a voltage in a fluid flowing through the tube. The voltage, which is proportional to flow velocity (rate of flow), is sensed by a pair of electrodes in contact with the fluid. The flow-dependent voltage is amplified and converted by a signal processing system to produce an output signal for display or control.
It is now common practice to provide ultrasonic cleaners on the electrodes of magnetic flowmeters, particularly when the meters are used for metering slow-moving liquids which have a tendency to coat the electrodes. The ultrasonic cleaners may, for example, consist of ultrasonic transducers in the form of piezoelectric disks which are positioned in or on the electrode bodies and which are energized periodically by an ultrasonic frequency generator. The electrodes are shaped to cause the vibration of the transducer to be transmitted to the face of the electrode in contact with the fluid in the body of the flowmeter and clean the electrode face. Examples of suitable electrodes are shown, for example, in U.S. Pat. Nos. 3,771,361 to Reznick, 3,479,873 to Hermanns, and 3,229,129 to van Haagen (using a magnetostrictive transducer).
In the operation of magnetic flowmeters having ultrasonic electrode cleaners, the high voltage ultrasonic energy utilized to activate the ultrasonic transducers is capable of interfering with the low level flow-dependent analog output produced across the electrodes. To overcome such interference, it has been common to provide low-pass filters in the signal processing circuitry. The low pass filters, however, have not been completely effective, and errors have been introduced into the output signal during the cleaning process and during a subsequent recovery period.